1. Field of the Invention
The present invention relates generally to batteries. More particularly, the present invention relates to a thin film lithium-ion battery.
2. Description of the Prior Art
Lithium-ion secondary batteries or lithium-ion batteries have been used as power supplies for personal computers, portable devices such as cell phones, cameras, electric tools, and the like. In secondary batteries, the electron producing and consuming reactions are for the most part reversible, and therefore such a battery can be cycled between a charged and discharged state electrochemically.
When the rechargeable battery is charged, ions formed of the cathode material pass from the cathode through the electrolyte to the anode, and when the battery is discharged these ions travel back from the anode through the electrolyte to the cathode. For example, in batteries having a cathode comprising lithium, such as a LiCoO2 or LiMnO2 cathode, lithium species originating from the lithium-containing cathode travel from the cathode to the anode and vice versa during the charging and discharging cycles, respectively.
FIG. 1 illustrates a conventional structure of a lithium-ion battery. As shown in FIG. 1, the lithium-ion battery 1 includes an electrochemical cell comprising an anode active material layer 11 disposed on one side surface of a separator 10, a cathode active material layer 21 disposed on the other side surface of the separator 10, an anode current collector 12, and a cathode current collector 22. The separator 10 may be made of polymers such as polyimide (PI), polypropylene (PP), polyethylene (PE), polyvinyl chloride (PVC) or polycarbonate (PC) having porous structure to only allow the passage of the lithium ions, while preventing internal shorting between the anode active material layer 11 and the cathode active material layer 21. To electrically connect the anode current collector 12 and the cathode current collector 22 to an external circuit or device, the lithium-ion battery 1 may further include two outwardly extended tabs 12a and 22a. 
Typically, the separator 10, the anode active material layer 11 and the cathode active material layer 21 are wetted with a liquid electrolyte solution or gel electrolyte. The electrochemical cell is typically enclosed in a parallelepipedic metal case 20 such as an aluminum case in a gas-tight manner with a sealant layer 24 securely sealing a gap between the tabs 12a and 22a. 
FIG. 2 illustrates another form of a lithium-ion battery known in the art. As shown in FIG. 2, the lithium-ion battery 3 is integrated with a circuit substrate 30 such as a copper clad laminate (CCL) substrate. The base dielectric of the CCL substrate may include polyimide (PI), polyethylene terephthalate (PET) or glass fiber. The circuit substrate 30 includes a separator portion 30a having therein a plurality of through holes or porous structures for the passage of lithium ions. The separator portion 30a is sandwiched by a pair of electrodes 41 and 51. A current collector 42 is disposed directly on a top surface of the electrode 41. The electrode 41 is sealed by a packaging unit 43. Likewise, a current collector 52 is disposed directly on a top surface of the electrode 51. The electrode 51 is sealed by a packaging unit 53. Both of the current collectors 42 and 52 are typically made of expensive CCL substrates. The use of CCL substrates increases manufacturing cost/complexity and battery weight.
Portable electronic devices have been progressively reduced in size and weight and improved in performance. It is therefore required to develop a rechargeable lithium-ion battery or lithium-ion secondary cell having a high energy density and a high output, which is also cost-effective. Further, after being stored or circled for certain numbers, gas may be generated in lithium-ion batteries, especially at high temperature, which will reduce life span of the lithium-ion battery. What is needed, therefore, is to provide a lithium-ion battery which has desirable life span.